1. Field of the Invention
The present disclosure relates to techniques for determining parameters of a wellsite. More particularly, the present disclosure relates to determining formation parameters, such as density, behind casing.
2. Background
Oil rigs may be positioned at wellsites for drilling a wellbore, performing downhole testing and producing located hydrocarbons. Downhole drilling tools may be advanced into the earth from a surface rig to form the wellbore. During drilling, measurements are often taken to determine downhole conditions. In some cases, the drilling tool may be removed so that a wireline testing tool may be lowered into the wellbore to take additional measurements and/or to sample downhole fluids. Once the drilling operation is complete, a casing may be positioned into the wellbore and cemented in place to line the well. Production equipment may also be positioned in the wellbore to assist in drawing the hydrocarbons from a subsurface reservoir to the surface.
In some cases, it may be necessary to take downhole measurements after the casing is installed in the wellbore. There may be reasons for making traditional ‘open hole’ measurements, such as those taken by the drilling and/or wireline tools, after casing has been set. Some of the reasons for taking these ‘cased hole’ measurements may include, for example, difficult logging conditions, highly deviated wells where deployment is a problem, or simply avoiding expensive rig time. The introduction of pulsed neutron capture measurements nearly 50 years ago provided some of the first opportunities to do formation evaluation in cased wellbores.
Over the years new techniques have been developed for taking downhole measurements as described, for example, in U.S. Pat. Nos. 7,215,125, 7,292,942, 7,522,471, 7,755,032, 7,902,496, and 20110056681. Some of these techniques may involve taking downhole measurements, such as density, in cased wellbores as described, for example, in U.S. Pat. No. 7,292,942.
Some ‘open hole’ measurements, such as gamma ray, neutron porosity, capture spectroscopy and/or natural gamma ray spectroscopy measurements, may be fairly adaptable to the cased hole environment, in some cases with minimal modifications. Some open hole acoustic logging techniques, such as full waveform sonic logs, slowness time coherence processing and resistivity logs, have been attempted in cased wells. Various techniques for identifying or quantifying gas using neutron or pulsed neutron tools have also been attempted, but may not be as robust as the standard neutron density crossover effect typically seen in open hole applications. Modifications to algorithms and/or measurement procedures (e.g., slower logging speeds) may be needed to adapt some ‘open hole’ measurements to cased hole applications.
In some cases, certain ‘open hole’ measurements may not be readily adaptable to cased hole applications. Certain cased hole measurements, such as density, spontaneous potential (SP), image logging and magnetic resonance, may be more difficult to make, may be more sensitive to the cased environment, may not be equivalent to corresponding ‘open hole’ measurements, and/or may not be possible in a cased hole environment (e.g. where a measurement device cannot measure through metal devices, such as casing). For example, density measurements in cased wellbores may be limited by the shallow depth of investigation of available gamma-gamma density tools and/or by environmental sensitivity. Achieving deeper readings may be limited by the physics of the measurement.
Despite the advancement of ‘cased hole’ measurements, there remains a need for providing accurate measurement of a wide variety of downhole parameters, such as density, in a cased wellbore. The invention is directed at providing this need.